Graphoid need to support Vega 3.0 graphs. Currently graphs use version tag to identify v1 vs v2. The v3 should be identified by the schema tag - if present, use it, otherwise fallback to the current implementation.
Looks like Vega is up to version 4 now, so it might be worthwhile to work on supporting that too.
(Also, when the upgrade happens, it might be worth it to copy all the new documentation locally to mediawiki.org, so that we don't fall behind when it changes. It's rather difficult to find docs for Vega 2.0 or 1.0 now.)
Version 4 hasn't changed much from v3, especially in terms of the graphs. So basically it will be v1.5, v2, and v3+, at least from the integration perspective. Copying v1.5 and v2 docs might be a good idea.